1. Field of the Invention
The present invention relates to a seam forming method using a sewing machine, and in particular, relates to a method of forming a seam on a thick material to be sewn, such as leather for a ball or the like, a tent, or a sailcloth, where the right sides of two materials are made to face each other, the edges of the materials are then sewn, and the sewn material is then unfolded so as to conceal the sewn edges.
2. Description of the Related Art
In balls used in soccer games or the like, in car seats, and in furniture seats, high loads are imposed on each seam, and the threads tend to loosen and the seam may split open. In order to prevent such splitting, each ball is manufactured by hand by double-thread sewing; however, such hand sewing is very inefficient. On the other hand, in the manufacturing of car seats and furniture seats, no specific measures to prevent the seam splitting have been provided.
When sewing leather, tents, sail material, or the like by using a sewing machine, the right sides of two materials are made to face each other, the edges of the materials are then sewn using a lockstitch sewing machine. When using the sewn material, the material is unfolded so as to conceal the sewn edges.
The fastening strength of the sewn portion depends on (i) tensile strength of each of the needle (i.e., upper) thread and the looper (i.e., lower) thread, (ii) frictional force at each knot between the needle and looper threads, (iii) frictional force between the needle and looper threads and the upper and lower materials (or cloths). Therefore, the fastening strength of the sewn portion is imposed on a single thread and thus depends on the strength of a single thread. In addition, when the sewn material is unfolded, the needle and looper threads may be loosened, each knot may be deformed, and the upper and lower materials may be compressed due to the needle and looper threads.
According to such phenomena, when the sewn material (or cloth) is unfolded, the contact faces between the upper and lower materials are open so that the needle and looper threads or knots are exposed to the surface of the material, thereby degrading the product quality.
In order to solve such problems, a sewing technique as shown in FIG. 10 has been proposed, in which a lockstitch sewing machine is used for forming a single stitch by (i) forming two forward stitches while forwarding the upper cloth F1 and the lower cloth F2 and (ii) forming one backward stitch (or backstitch), and repeating this operation so as to form a seam.
The seam formed by the above method includes a hitch stitch H at which the needle thread A is kinked, where the hitch stitch H is produced when the backward stitching is switched to the forward stitching. In addition, the cloths are fastened by three needle threads A and three looper threads B. Therefore, the strength of pressing the cloths is improved, and a loose or split seam is less frequently produced. However, the stitch forming operation includes backstitching; thus, it is not easy for the operator to handle the cloths and the operator may experience discomfort.